1. Field of the Invention
The invention relates to a mixer, and more particularly, to a passive harmonic switch mixer for use in a direct conversion receiver or a direct conversion transmitter.
2. Description of the Prior Art
FIG. 1 shows a conventional passive mixer 10. The mixer, which is for mixing a differential RF signal pair RF+ and RF− with a differential local oscillator signal to output a differential baseband signal pair BB+ and BB−, is a very important component in communication systems. There are two types of mixers: the active mixer and the passive mixer. The conventional passive mixer 10, shown in FIG. 1, includes a first input port A and a second input port B for receiving a differential RF signal pair RF+ and RF− respectively and a first output port C and a second output port D for outputting a differential baseband signal pair BB+ and BB− respectively. A positive side capacitor 12 is connected to the node A and a negative side capacitor 22 is connected to the node B for DC isolation. The conventional passive mixer 10 includes four switch devices 14, 16, 18, 20, which are for mixing a differential RF signal RF+ and RF− with a differential local oscillator signal LO+ and LO−. The first switch device 14 selectively connects/disconnects the node A to a node C depending on the positive local oscillator signal LO+, the second switch device 16 selectively connects/disconnects the node A to a node D depending on the negative local oscillator signal LO−, the third switch device 18 selectively connects/disconnects the node B to the node C depending on the LO− signal, and the fourth switch device 20 selectively connects/disconnects the node B to the node D depending on the LO+ signal.
FIG. 2 shows a signal diagram 30 of the positive and negative local oscillator signal LO+ and LO−. The operation of the passive mixer 10 in FIG. 1 is well known to a person skilled in the art and can be summarized as repeatedly switching on and off a differential RF signal pair depending on a differential LO signal pair, which is shown in FIG. 2. The RF signal pair received on the RF port is mixed with the local oscillator signal pair received at the nodes A and B and the result of this mixing is seen as a baseband signal pair output on the nodes C and D.
In addition to the passive-type mixer circuit shown in FIG. 1, there are also various active-type mixer circuit designs well known to people skilled in the art. For example, Gilbert disclosed a conventional active-type mixer circuit design, which is commonly called the “Gilbert cell”, in U.S. Pat. No. 3,241,078 and various improvements based on “Gilbert cell” are disclosed afterward. The main advantage of the active mixer is the addition of signal gain to the output signal; however, it has the disadvantage that the low frequency flicker noise problem is severe.
There are two kinds of transceiver architectures used in modern communication systems, specifically the super heterodyne system and the direct conversion system. A detailed description of the transceiver architectures is disclosed in Behzad Razavi, “RF Microelectronics”, 1998, which is incorporated herein by reference.
FIG. 3 shows a typical application of a mixer circuit in a super heterodyne receiver 40, also known as an IF receiver. The super heterodyne receiver 40 includes a differential RF input port, a first band pass filter 42, a first low-noise amplifier 44, a second band pass filter 46, a first mixer 48, a first local oscillator 50, a third band pass filter 52, a second low-noise amplifier 54, a second mixer 56, a third mixer 57, a second local oscillator 58, and a 90° phase shifter 59.
The operational of the super heterodyne receiver 40 includes demodulation, carrier-frequency tuning, filtering, and amplification. An incoming RF signal is received at the RF input port, filtered by the first band pass filter 42, amplified by the low noise amplifier 44, and filtered again by the second band pass filter 46. As an example, in the case of IEEE802.11B WLAN, the first band pass filter 42, the first low-noise amplifier 44, and the second band pass filter 46 are configured for operation from 2.4 GHz to 2.48 GHz. Signals in this frequency range are amplified and allowed to pass to the first mixer 48 where they are mixed with the output from the first local oscillator 50. In the example case of 802.11B WLAN, the first local oscillator 50 operates at 2.076 GHz and the output of the first mixer 48 is at 378 MHz. In order to choose the desired channel, the output of the first mixer 48 is filtered by the third band pass filter 52, which operates as a SAW filter for channel selection. The second low noise amplifier 54 amplifies the output of the third band pass filter 52 and outputs the amplified signal to both the second mixer 56 through the I-pathway and the third mixer 57 through the Q-pathway. Both the second mixer 56 and the third mixer 57 are for mixing the amplified signal output from the second low noise amplifier 54 with the output from the second local oscillator 58. Continuing the 802.11B WLAN example, the second local oscillator 58 provides a 374 MHz local oscillator signal to the second mixer 56 directly. However, the phase of the local oscillator signal must be 90° shifted by the 90° phase shifter 59 and then input to the third mixer 57. The output of the second mixer 56 and the third mixer 57 are a differential in-phase baseband signal and a differential quadrature-phase baseband signal respectively.
In order to reduce the size, cost, and power consumption of modern communication systems, the trend in todays increasingly wireless and mobile society is to embed complete systems onto a single integrated circuit (IC). Although the super heterodyne receiver provides a well-known solution for the reception and demodulation of RF signals, its implementation requires the use of many components that are fabricated external to the IC. Specifically, due to the large area required to fabricate internal to an IC, the band pass filters 42, 46, 52 and the local oscillators 50, 58 are all external components. In this manner, the circuit design is complicated by the difficult to implement impedance-matching issues.
FIG. 4 shows a direct conversion receiver 60 according to the conventional art. By directly mixing the received RF signal with a local oscillator signal running at the same frequency with the RF signal, the baseband signal can be recovered in a single step. The direct conversion receiver 60 is also known as a zero IF receiver or a homodyne receiver and includes an input RF port connected to a band pass filter 62. The output of the band pass filter 62 is connected to a low noise amplifier 64 and the output of the low noise amplifier 44 is connected to both a first mixer 66 and a second mixer 68. A local oscillator 68 provides a local oscillator signal connected to the first mixer 66 directly. However, the phase of the local oscillator signal must be 90° shifted by the phase shifter 69 and then input to the second mixer 70. The output of the first mixer 66 and the second mixer 70 are a differential in-phase baseband signal and a differential quadrature-phase baseband signal respectively.
The operation of the direct conversion receiver 60 includes demodulation and amplification. An incoming RF signal is received at the input RF port, filtered by the band pass filter 62, and amplified by the low noise amplifier 64. Using the 802.11B WLAN example, the incoming RF signal, the band pass filter 62, and the low noise amplifier 64 operate from 2.4 GHz to 2.48 GHz. The output of the low noise amplifier 64 is connected to both the first mixer 66 and the second mixer 70. In the case of 802.11B WLAN, the local oscillator also operates at 2.4 GHz allowing direct recovery of the baseband signal on the BB port.
Although the direct conversion receiver 60 reduces the required external components to the band pass filter 62 and the local oscillator 68, additional problems are encountered. The direct conversion receiver 60 suffers from leakage noise and flicker noise problems, both reducing the overall signal to noise ratio (SNR) of the direct conversion receiver 60. A DC offset on the baseband signal output on the BB port is ultimately caused because the local oscillator operates at 2.4 GHz, which is the same with the RF signal. Therefore, the isolation between the local oscillator 68 and the inputs to the mixer 66 as well as the input to the low noise amplifier 64 is not perfect. Feed through, also referred to as leakage noise, from the local oscillator 68 to the input of the low noise amplifier 64 or the mixer 66 is mixed with the original local oscillator signal causing a DC offset voltage to appear on the baseband signal at the BB port. Additionally, flicker noise, which is caused by the input noise of a transistor logic component and is inversely proportional with frequency, reduces the signal to noise ratio seen at the output BB port.
The direct conversion receiver, as well as a direct conversion transmitter, suffer from the same leakage noise, DC offset, and flicker noise problems. The problems are due to the strong RF signal being centered at the same frequency as the local oscillator signal and a portion of this signal being leaked back to the local oscillator and injected into the mixer where it is mixed with the local oscillator signal. This injection occurs due to non-ideal isolation between the local oscillator and the amplifier. Even if careful shielding techniques are used, there is still a finite amount of radiation and or conduction of the RF output signal back to the local oscillator. In addition, the direct conversion transmitter also suffers from pulling noise resulting from the voltage variation of VDD and Ground during ON and OFF switching of the power amplifier. Pulling noise may cause serve frequency shifting of the power amplifier in the direct conversion transmitter.